1. Field of the Invention
This invention relates generally to the fabrication of thin films by ion beam sputter deposition and, more particularly, to the fabrication of multilayered thin film structures such as magnetoresistive sensors by ion beam sputter deposition wherein the properties of multiple layers deposited on a substrate are controlled by controlling the angle at which atoms are deposited on the substrate.
2. Description of Related Art
It is well-known in the art to utilize radio frequency (RF) or direct current (DC) magnetron sputter-deposition system for fabrication of thin film devices such as magnetic recording sensors (e.g., magnetoresistive sensors) and storage media. Such sputter-deposition systems are characterized by crossed electric and magnetic fields in an evacuated chamber into which an inert, ionizable gas, such as argon, is introduced. The gas is ionized by electrons accelerated by the electric field, which forms a plasma in proximity to a target structure. The crossed electric and magnetic fields confine the electrons in a zone between the target and substrate structures. The gas ions strike the target structure, causing ejection of atoms that are incident on a workpiece, typically a wafer substrate on which it is desired to deposit one or more layers of selected target materials.
In the conventional sputtering deposition systems relatively high operating pressures are utilized in order to obtain films having low internal stress which results in non-directional sputtering flux at the substrate. However, this non-directional flux introduces manufacturing process difficulties as device dimensions become increasingly smaller.
It is known to utilize ion beam sputter deposition in certain applications to overcome some of the difficulties encountered with conventional RF/DC sputter techniques. Several aspects of ion beam sputter deposition systems differ from conventional sputter deposition systems and provide significant advantages. For example, (1) the use of low background pressure results in less scattering of sputtered particles during the transit from the target to the wafer substrate; (2) control of the ion beam directionality provides a variable angle of incidence of the beam at the target; (3) a nearly monoenergetic beam having a narrow energy distribution provides control of the sputter yield and deposition process as a function of ion energy and enables accurate beam focusing and scanning; and (4) the ion beam is independent of target and substrate processes which allows changes in target and substrate materials and geometry while maintaining constant beam characteristics and allowing independent control of the beam energy and current density.
Apparatus and methods for depositing a thin layer of material on a substrate utilizing ion beam sputtering deposition systems are described, for example, in U.S. Pat. No. 4,923,585 ('585) to Krauss et al. and in U.S. Pat. No. 5,942,605 to Pinarbasi ('605), the contents of which are incorporated herein by reference. The '585 patent discloses the use of a computer controlled, single ion beam with a quartz crystal monitor to produce deposited films of arbitrary composition as well as layered structures of arbitrary thickness from multiple targets of different materials. The '605 patent discloses matching the ion beam gas atomic mass to the target material atomic mass to produce thin films having densities and physical properties very close to their bulk property values. While the '585 and '605 patents disclose methods for depositing multilayer films, the problems of controlling the amount of flux deposited at the junction between the layers deposited adjacent to each other is not addressed.
Ion beam sputter deposition systems have been utilized to deposit individual layers of anisotropic magnetoresistive (AMR) sensors and giant magnetoresistive (GMR) sensors for use in magnetic disk drives. In the GMR sensors, for example, the resistance of the magnetoresistive (MR) sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between the ferromagnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the ferromagnetic and non-magnetic layers and within the ferromagnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., NiFe or Co or NiFe/Co) separated by a layer of GMR promoting non-magnetic metallic material (e.g., copper) are generally referred to as spin valve (SV) sensors. U.S. Pat. No. 5,206,590 to Dieny et al. ('590), the content of which is incorporated herein by reference, discloses an MR sensor operating on the principle of GMR.
Magnetoresistive (MR) sensors (AMR or GMR) are very small devices that are generally fabricated by sputtering depositions on large wafer substrates which are generally larger than 5 inches in diameter to form thousands of sensors. The wafer is subsequently diced to form individual magnetic read transducers for use in magnetic storage devices.
One of the major issues in the fabrication process of MR sensors is to precisely control the physical, electrical and magnetic properties of the junction formed between the layers deposited adjacent to each other. An example of such a junction is the contiguous junction formed between the MR layer and the longitudinal biasing layer in an MR sensor.
Another critical issue in the fabrication process of MR sensors is the thickness uniformity of each and every deposited layer over the entire utilized area of a given wafer in order to control the uniformity of operating characteristics (for example, resistance and magnetoresistance) of the entire batch of the MR sensors fabricated on said given wafer.
In an experiment by the present applicant, an ion beam sputtering system 120 (FIG. 1) was developed and used to determine the properties of the junction formed between the layers deposited adjacent to each other and thickness uniformity of various layers deposited in the end region 206 and 204 of the SV sensor 200 (FIG. 2A) formed on a 5 inch diameter wafer substrate (FIG. 3).
Referring to FIG. 1, there is shown a simplified diagram illustrating the ion beam sputter deposition system 120 developed and used by the Applicant. The ion beam sputter deposition system 120 includes a vacuum chamber 122 in which an ion beam source 121 is mounted. The ion beam system 120 further comprises selectable multiple targets 123, formed or mounted, on a rotary target support 125. An ion beam 133 provided by the ion beam source 121 is directed at one of the targets on the selectable multiple targets 123 where the impacting ions cause sputtering of the selected target material. The sputtered atoms 126 emitted from the selected target material is directed at a near-normal angle (85 to 95 degrees) onto a workpiece (wafer substrate, wafer, deposition substrate) 131 on which is formed a layer of the selected target material. The sputtered atoms 126 hit (bombard) the workpiece 131 at a near-normal angle (85 to 95 degrees). The workpiece 131 is placed securely, via clamps or vacuum suction (not shown) on a substrate stage (workpiece stage) 141. The substrate stage 141 is retrievable into a loading port 139 via a gate valve 138 for changing the workpiece 131.
A thickness monitor 137, positioned closely adjacent to the workpiece 131, provides real-time, in-situ monitoring of the thickness of the growing film during deposition over the entire utilized area of the workpiece 131. A non-movable flux regulator 150 fixed in front of the workpiece 131 partially blocks the sputtered atom flux and is used in conjunction with rotation of the workpiece 131 to improve thickness uniformity of the deposited layer during the deposition process. The non-movable flux regulator refers to a flux regulator that its position is fixed prior to the ion beam sputtering deposition of one or more deposited layers and remains fixed during the whole deposition process (i.e., the position of the flux regulator is never changed during the deposition process of said one or more deposited layers). During operation of the ion beam sputter deposition system, the vacuum chamber 122 is maintained at suitable low pressure by a vacuum pump (not shown) via port 135.
Now referring to FIG. 2A, there is shown a cross section of the SV sensor 200 having end regions 204 and 206 separated from each other by a central region 202 where Applicant's ion beam sputtering system 120 was used to deposit seed, biasing and lead layers in said end regions. A free layer (free MR layer, free ferromagnetic layer) 210 is separated from a pinned layer (pinned MR layer, pinned ferromagnetic layer) 220 by a non-magnetic, electrically-conducting spacer layer 215. Alternatively, pinned layer 220 may be made of multi layers of ferromagnetic material (e.g., cobalt, Nife) separated from each other by a metallic non-magnetic conductor (e.g., ruthenium). Such a multi-layer pinned layer is generally referred to as anti-parallel (AP) pinned layer. The magnetization of the pinned layer 220 is generally, although not necessarily, is fixed (i.e., pinned) through exchange coupling with an antiferromagnetic (AFM) layer 225. The AFM layer 225, is generally made of NiMn, FeMn or NiO. The magnetization of the free layer, however, is free to rotate in response to an external field. Free layer 210, spacer layer 215, pinned layer 220 and the AFM layer 225 (if used), which are collectively referred to as MR material, are all formed in the central region 202 over the substrate 228. Hard bias (HB) layers 230 and 235 which are formed in the end regions 204 and 206, respectively, provide longitudinal bias for the MR free layer 210. Hard bias layers 230 and 235 are generally, although not necessarily, deposited over seed layers 280 and 285, respectively. Hard bias layers 230 and 235 form contiguous junctions 274 and 276, respectively, with at least the free layer 210. Leads 240 and 245 which are formed over the hard bias layers 230 and 235, respectively, provide electrical connections for the flow of the sensing current I.sub.s from the current source 260 to the MR sensor 200. The MR material further has first and second side edges 270 and 272 (FIG. 2B).
FIG. 3 shows a wafer 300 manufactured by Applicant's ion beam sputtering system for making SV sensors. FIG. 3 illustrates schematically the general pattern of several blocks 301, each block comprising a plurality of rows 302. Each of the rows 302 comprises a plurality of SV sensors (such as SV sensor 200 or 900) disposed along each row and formed on the wafer substrate 306.
As mentioned earlier, Applicant conducted an experiment in which the ion beam sputtering system 120 was used to build SV sensors 200 on the wafer substrate 306. In doing so, layers of sputtered material comprising the layer structure of the central region 202 of the SV sensor 200 were individually deposited on the whole wafer. Photoresist materials 290 and 291 were then deposited on the whole wafer, after which they were exposed to light in selected regions and developed to provide openings for removal of the deposited materials outside of the central region 202. FIG. 2B shows the step in the manufacturing process of the SV sensor 200 after photoresists 290 and 291 have been developed and the deposited materials outside of the central region 202 have been removed using ion-milling. Following the step shown in FIG. 2B, seed layer material, hard bias material and lead materials were sputtered deposited sequentially in the end regions 204 and 206. The materials deposited in the end regions 204 and 206 were sputtered deposited at near-normal (85 to 95 degree) angle as shown by arrows 292 (FIG. 2C).
Close examination of the SV sensor 200 (FIG. 2A) reveals the following shortcomings present in the SV sensor 200 formed according to the aforementioned steps:
(i) the thickness of the seed layers 280 and 285 are not uniform; PA1 (ii) the thickness of the hard bias layers 230 and 235 are not uniform; hard bias layers 230 and 235 taper off by the first and second side edges 270 and 272, respectively; and a notch is formed in each of the hard bias layers 230 and 235 adjacent to the MR material side edges 270 and 272. Hard bias tapering results in low coercivity hard bias material deposited at the edges of the MR material resulting in MR sensor instability during the read operation; and PA1 (iii) the thickness of the lead layers 240 and 245 are not uniform and they taper off near the MR material side edges 270 and 272. Lead tapering results in loss of electrical signal. PA1 (i) lack of seed layer, hard biasing layer and lead layer thickness uniformities in each MR sensor built in an ion beam sputter deposition system; PA1 (ii) poor physical, electrical and magnetic properties at the contiguous junction between materials deposited adjacent to each other; and PA1 (iii) lack of thickness uniformity of thin layers deposited over the entire utilized area of the wafer in an ion beam deposition system.
Furthermore, Applicant conducted another experiment in which the ion beam sputtering system 120 was used to build SV sensors 200 on the utilized area of the wafer substrate 306 and the uniformity of the deposited layers across the entire utilized area of the wafer were measured by measuring the sheet resistance of each deposited layer at the five positions 305 indicated on a diameter of the wafer 300. The uniformity of the sheet resistance across the wafer is expressed as a percent uniformity which is a measure of the maximum variation in the film thickness that may be seen on a given wafer.
FIG. 4 is a graph showing the normalized sheet resistance of the Cu spacer film 215 of the SV sensor 200 measured at five positions 305 across the wafer 300. As shown in FIG. 4, the Cu film thickness varied by as much as 11.3% across the utilized area of the wafer 300. The NiFe and Co layer thicknesses, measured at the same five locations 305 across the wafer 300, varied by about 3.5% and 2.7%, respectively.
Eleven and three tenth percent (11.3%) variation in the thickness of the Cu spacer film across the wafer 300 means that many of the MR sensors on wafer 300 fail to work properly or have unacceptably large variations in their responses. Also, as the size of the wafer increases to improve productivity, the problem of achieving film uniformity across the wafer becomes even worse.
The prior art does not address or acknowledge the problems associated with:
Therefore, there is a need for an invention of a method and apparatus for controlling the properties and thickness of individual films of a multilayer thin film structure deposited adjacent to each other on a wafer substrate in an ion beam sputtering system.
There is a further need for an invention of a method and apparatus for controlling the thickness uniformity of individual films of a multilayer thin film structure deposited on a wafer substrate in an ion beam sputtering system.
There is also a further need for an invention disclosing a method and apparatus to control the thickness of individual layers of the multilayer structures of MR sensors deposited adjacent to each other on a wafer substrate in an ion beam sputtering system.